Some gold-bearing sulfide ores are refractory to gold recovery because all or a significant portion of the gold is locked within sulfide minerals. Consequently, these ores, or concentrates made from such ores, are often subjected to a pretreatment process before leaching the gold with a leach solution containing cyanide or some other gold lixiviant. Such pretreatment processes generally involve oxidization of sulfide sulfur contained in the sulfide minerals, thereby decomposing the sulfide minerals and releasing gold for recovery.
One pretreatment process is biooxidation. Biooxidation pretreatment involves the use of iron-oxidizing acidophilic microorganisms to promote oxidation of the sulfide minerals in which gold is locked, releasing the gold for recovery. Biooxidation pretreatment may be performed in heaps, vats, tanks, or other reactor systems.
Acidophilic microorganisms used during biooxidation pretreatment thrive in a highly acidic environment, and the effectiveness of the biooxidation pretreatment partly depends on an ability to maintain suitable highly acidic conditions in the reactor system to promote growth of the microorganisms. Some gold sulfide ores, however, contain a significant amount of acid-consuming minerals, and particularly acid-consuming carbonate minerals such as for example calcite and/or dolomite. These acid-consuming components tend to react with and neutralize available acid, which can seriously interfere with establishment and maintenance of desired acidic conditions conducive to microbial growth for effective biooxidation pretreatment.
A biooxidation pretreatment operation may be able to accommodate acid consumption associated with a low concentration of acid-consuming components, but as the concentration of acid-consuming minerals increases, it may become necessary to preacidify an ore material, such as by the addition of sulfuric acid, prior to conducting the biooxidation pretreatment. Another possibility is to attempt to physically remove acid-consuming components prior to the biooxidation pretreatment. For example, an ore might be subjected to flotation in an attempt to prepare a sulfide concentrate that is largely free of acid-consuming components. Because of the substantial costs associated with pre-acidification and flotation operations, however, those techniques are often not economically feasible for treating ore materials with lower gold grades.